The invention relates to a method and an apparatus for demodulating a TFM-modulated signal comprising symbols. The invention is particularly applicable in radio systems that employ the TFM modulation method.
A modulation method to be used on the transmission path is an essential parameter in data transmission systems, particularly in radio systems. Information symbols to be transferred cannot be transferred as such over the transmission path owing to the losses occurring on the transmission path and to the capacity of the transmission path. Instead the symbols must be modulated using an appropriate method in order to obtain an adequate capacity and quality on the transmission path.
Particularly in radio systems an essential factor is the band-width required by the transmission. It is an aim to achieve a transmission capacity that is as extensive as possible on a limited bandwidth. On the other hand, both the transmitter and the receiver should be as simple as possible and advantageous to implement. What is mainly employed in radio systems is a standard envelope modulation method, since then a C-class amplifier solution can be used. The structure of the C-class amplifiers is simple and the efficiency thereof is advantageous. This is substantial particularly in view of the power consumption of the terminals.
Several known modulation methods exist that have a constant envelope. One example is the CPM (Continuous Phase Modulation) type of methods, such as MSK (Minimum Shift Keying), GMSK (Gaussian Minimum Shift Keying), TFM (Tamed Frequency Modulation). The TFM is a particularly interesting modulation method, since the spectrum use thereof is efficient and also as the TFM modulator can be implemented using a simple FM modulator. Consequently the transmitter could be implemented in a very simple manner.
The FM modulator is thus a preferable way to implement a CPM signal. However, keeping the modulation index constant during transmission constitutes a problem for a wider use of the FM modulator. In the TFM method, the modulation index should be constant, and determined to have the value 0.5. When using the FM modulator the variation of the modulation index causes time varying phase jitter to the signal to be transmitted. Since the phase of a generated TFM signal cannot be kept at a desired level during transmission, the performance deteriorates significantly in a coherent receiver.
In prior art solutions; a suggestion has been made to implement compensation for the modulation index variation in connection with the transmitter. This is explained, for instance, in publication Jager F. and Dekker C. D., A Novel Method to Achieve Spectrum Economy in Digital Transmission. IEEE Transactions on Communications, May 1978, Vol. 26, No. 5, pages 534 to 542. A drawback with this system is that the structure of the transmitter becomes complicated.
Publication Fonollosa J. R. and Fonollosa J. A. R., Estimation of the modulation index of CPM signals using higher-order statistics. Proc. of ICASSP 93, April 27-30, Minneapolis, Minn., pages IV 268 to 271 discloses a solution, in which the modulation index variation is compensated in the receiver, but the solution either requires determining the theoretical autocorrelation or measuring the cross-correlation in the receiver. These are demanding tasks and the latter requires the existence of known data, such as a training sequence.
Publication Miller M. J., Detection of CPFSK Signal Using Per Survivor Processing. Proceedingsxe2x80x94IEEE Military Communications Conference MILCOM, Oct. 19-21, 1998, Bedford, Mass., USA, pages 524 to 528 shows that the per-survivor processing, PSP, phase synchronization algorithm of the carrier wave is used in the receiver to compensate for the phase jitter caused by the modulation index variation. However, if the deviation of the modulation index between the receiver and the transmitter is high, then a mere phase synchronization algorithm is not enough.
It is an object of the present invention to provide a method and an apparatus implementing the method in such a manner that the modulation index variation caused by an FM modulator can be compensated in a demodulator. This is achieved with a demodulation method for a TFM-modulated signal comprising symbols, the method comprising the steps of calculating an estimate for the symbols, determining an estimate for the phase shift, determining a modulation index estimate error using the phase shift and the symbol estimates, removing the.estimated error from the modulation index estimate.
The invention also relates to a demodulator for a TFM-modulated signal comprising symbols, the demodulator comprising means for calculating an estimate for the symbols, means for determining an estimate for the phase shift, means for determining a modulation index estimate error using the phase shift and the symbol estimates and means for removing the estimated error from the modulation index estimate.
The preferred embodiments of the invention are disclosed in the dependent claims.
In the preferred embodiments of the invention the modulation index variation can be compensated in the receiver. Thus, a simple FM modulator can be used as the transmitter. This provides significant advantages in several telecommunication systems, for example in point-to-multipoint (PMP) systems that include one central station and a large number of terminals. The structure of the central station may be complex, but the simple structure of the terminals provides significant savings. The present invention makes this possible.
In the preferred embodiments of the invention, the modulation index variation is based on a TFM coding rule and on the symbol decisions made. The value of the modulation index can be estimated continuously and the estimation allows making the modulation index in the receiver congruent with the modulation index of the transmitter. In a preferred embodiment the index estimated in the receiver is sent to the transmitter, which may correct the modulation index in accordance with the information obtained to be closer to the defined nominal value 0.5.